1. Field of the Disclosure
The present disclosure relates to radio broadcast receivers and, in particular, to methods and systems for recovering a DC component in a zero intermediate frequency (referred to herein as zero-IF) radio receiver.
2. Background Information
Digital radio broadcasting technology delivers digital audio and data services to mobile, portable, and fixed receivers. One type of digital radio broadcasting, referred to as in-band on-channel (IBOC) digital radio broadcasting, uses terrestrial transmitters in the existing Medium Frequency (MF) and Very High Frequency (VHF) radio bands. HD Radio™ technology, developed by iBiquity Digital Corporation, is one example of an IBOC implementation for digital radio broadcasting and reception.
IBOC digital radio broadcasting signals can be transmitted in a hybrid format including an analog modulated carrier in combination with a plurality of digitally modulated carriers or in an all-digital format wherein the analog modulated carrier is not used. Using the hybrid mode, broadcasters may continue to transmit analog AM and FM simultaneously with higher-quality and more robust digital signals, allowing themselves and their listeners to convert from analog-to-digital radio while maintaining their current frequency allocations.
One feature of digital transmission systems is the inherent ability to simultaneously transmit both digitized audio and data. Thus the technology also allows for wireless data services from AM and FM radio stations. The broadcast signals can include metadata, such as the artist, song title, or station call letters. Special messages about events, traffic, and weather can also be included. For example, traffic information, weather forecasts, news, and sports scores can all be scrolled across a radio receiver's display while the user listens to a radio station.
IBOC DAB technology can provide digital quality audio, superior to existing analog broadcasting formats. Because each IBOC DAB signal is transmitted within the spectral mask of an existing AM or FM channel allocation, it requires no new spectral allocations. IBOC digital radio broadcasting promotes economy of spectrum while enabling broadcasters to supply digital quality audio to the present base of listeners.
Multicasting, the ability to deliver several audio programs or streams over one channel in the AM or FM spectrum, enables stations to broadcast multiple streams on separate supplemental or sub-channels of the main frequency. For example, multiple streams of data can include alternative music formats, local traffic, weather, news, and sports. The supplemental channels can be accessed in the same manner as the traditional station frequency using tuning or seeking functions. For example, if the analog modulated signal is centered at 94.1 MHz, the same broadcast in IBOC digital radio broadcasting can include supplemental channels 94.1-1, 94.1-2, and 94.1-3. Highly specialized programming on supplemental channels can be delivered to tightly targeted audiences, creating more opportunities for advertisers to integrate their brand with program content. As used herein, multicasting includes the transmission of one or more programs in a single digital radio broadcasting channel or on a single digital radio broadcasting signal. Multicast content over IBOC digital radio broadcasting transmissions can include a main program service (MPS), supplemental program services (SPS), program service data (PSD), and/or other broadcast data.
The National Radio Systems Committee, a standard-setting organization sponsored by the National Association of Broadcasters and the Consumer Electronics Association, adopted an IBOC standard, designated NRSC-5A, in September 2005. NRSC-5A, the disclosure of which is incorporated herein by reference, sets forth the requirements for broadcasting digital audio and ancillary data over AM and FM broadcast channels. The standard and its reference documents contain detailed explanations of the RF/transmission subsystem and the transport and service multiplex subsystems. Copies of the standard can be obtained from the NRSC at http://www.nrscstandards.org/standards.asp. iBiquity's HD Radio technology is an implementation of the NRSC-5A IBOC standard. Further information regarding HD Radio technology can be found at www.hdradio.com and www.ibiquity.com.
Other types of digital radio broadcasting systems include satellite systems such as Satellite Digital Audio Radio Service (SDARS, e.g., XM Radio™, Sirius®), Digital Audio Radio Service (DARS, e.g., WorldSpace®), and terrestrial systems such as Digital Radio Mondiale (DRM), Eureka 147 (branded as DAB Digital Audio Broadcasting®), DAB Version 2, and FMeXtra®. As used herein, the phrase “digital radio broadcasting” encompasses digital audio and data broadcasting including in-band on-channel broadcasting, as well as other digital terrestrial broadcasting and satellite broadcasting.
Some modern radio receivers use a zero-IF architecture (sometimes referred to as direct conversion) in which a signal is frequency down-converted in mixers directly to quadrature related in-phase (I) and quadrature (Q) signals at baseband frequencies. These I and Q signals are then applied to a demodulator in which the modulation in the signal is recovered. Zero-IF receivers can provide a number of advantages. For example, converting the incoming radio frequency (RF) signal directly to baseband allows channel amplification and filtering to be performed at baseband frequency, which facilitates integration of the receiver with other components. Additionally, zero-IF receivers may have lower gate counts, thereby resulting in lower power consumption and lower cost. The present inventors have observed that it would be desirable to utilize a zero-IF architecture in, for example, digital radio broadcast receivers that include analog FM receiver sections. It can also be used in analog FM-only receivers that use digital techniques for demodulation.
However, a problem with zero-IF receivers is that an unwanted DC voltage can be created at the mixer output in addition to the desired baseband signal. This unwanted DC voltage results from reverse-transmission paths in the tuner circuitry that occur when local oscillator (LO) energy leaks through the mixer, thereby causing the LO energy to self-mix, and also from the 2nd order nonlinearity of the mixer and baseband amplifiers when the signal level is high. This unwanted DC voltage can create distortions in the baseband signal and so typically must be removed by placing a DC blocking filter in the tuner circuitry after the mixer. However, this approach typically removes not only the unwanted DC voltage, but also any legitimate DC component of the baseband signal. This removal of the DC component from the baseband signal can result in unacceptable distortions in the demodulated signal. As a result, the use of a zero-IF architecture has typically been restricted to applications that do not involve reception of signals having a significant DC component. This means that a zero-IF architecture typically has not been used in, for example, analog FM broadcast receivers or in the analog FM portion of digital FM radio broadcast receivers. Rather, such digital radio broadcast receivers have included a low-IF receiver section for receiving both digital FM and analog FM signals, or a zero-IF receiver section for receiving the digital FM signals and a low-IF receiver section for receiving the FM analog signals, wherein the receiver bifurcates the incoming signal to these two sections for demodulation. The present inventors have observed a need for methods and systems that permit receiving an analog FM signal using a zero-IF architecture, preferably the same zero-IF architecture used for receiving digital radio broadcast signals.